Intrinsic Out-Of-Plane and In-Plane Ferroelectricity in 2D AgCrS with High Curie Temperature.

2D ferroelectric materials have attracted extensive research interest due to potential applications in nonvolatile memory, nanoelectronics and optoelectronics. However, the available 2D ferroelectric materials are scarce and most of them are limited by the uncontrollable preparation. Herein, a novel 2D ferroelectric material AgCrS is reported that are controllably synthesized in large-scale via salt-assist chemical vapor deposition growth. By tuning the growth temperature from 800 to 900 °C, the thickness of AgCrS nanosheets can be precisely modulated from 2.1 to 40 nm. Structural and nonlinear optical characterizations demonstrate that AgCrS nanosheet crystallizes in a non-centrosymmetric structure with high crystallinity and remarkable air stability. As a result, AgCrS of various thicknesses display robust ferroelectric polarization in both in-plane (IP) and out-of-plane (OOP) directions with strong intercorrelation and high ferroelectric phase transition temperature (682 K). Theoretical calculations suggest that the ferroelectricity in AgCrS originates from the displacement of Ag atoms in AgS tetrahedrons, which changes the dipole moment alignment. Moreover, ferroelectric switching is demonstrated in both lateral and vertical AgCrS devices, which exhibit exotic nonvolatile memory behavior with distinct high and low resistance states. This study expands the scope of 2D ferroelectric materials and facilitates the ferroelectric-based nonvolatile memory applications.